As a rule, hydrogen peroxide is produced by the so-called anthraquinone process by alternate reduction and oxidation of alkylated anthraquinones dissolved in suitable organic solvents. The solution of anthraquinones, the so-called working solution, is first treated with hydrogen gas in the presence of a catalyst in the so-called hydrogenation step. Then the working solution is conducted to the so-called oxidation step in which it is contacted with air or oxygen-containing gas to form hydrogen peroxide. One example of a reaction scheme for these hydrogenation and oxidation steps is ##STR1##
The hydrogen peroxide containing working solution is then usually conducted to a so-called extraction step in which the hydrogen peroxide is removed from the working solution by extraction with water, whereupon the working solution is recycled to the hydrogenation step for a further cycle of the above-mentioned reaction steps.
The hydrogenation is an important step of this continuous process, and a number of difficulties are here encountered. During hydrogenation there are high demands for a hiigh and uniform productivity, but also for the selectivity of the reaction to avoid secondary reactions that may prevent or aggravate the formation of hydrogen peroxide. The selectivity depends on a number of factors, among them the reaction degree of the anthraquinones, the hydrogenation temperature, the hydrogen gas pressure, the catalyst, and the flow conditions in the catalyst.
A common technique is hydrogenation with the catalyst in suspended form, whereby a good contact between the three reacting phases and thus high productivity and selectivity are obtained. Frequently, however, filtration difficulties arise with this technique when the finished hydrogenated solution is to be conducted to the next process step, in that the filters are easily blocked by, for example, catalyst particles, or in that leakage of particles occurs. The filters employed are usually expensive and require great care.
To avoid such filtration difficulties, the catalyst may be applied on a fixed bed, a so-called fixed bed hydrogenation. In this manner, the filtration difficulties are avoided, but the productivity and selectivity results will frequently be inferior to those obtained with suspended catalyst hydrogenation. The reason for this is an inferior contact between the three reacting phases because of an uneven flow distribution (so-called channeling) and a high pressure drop across the bed.
One way of improving the fixed bed hydrogenation is to arrange the fixed bed in the form of similar, parallel channels, a so-called monolithic fixed bed (see EP patent specification 102,934). In this manner, the contact between the three phases will be improved, the flow distribution will be more even, and the pressure drop lower.